The present invention relates to a semiconductor device and a method of forming the same, and more particularly to a life-time improved gallium nitride based semiconductor laser diode with a gallium nitride based active layer and a method of forming the same.
A nitride semiconductor layer of a luminescent device has many through dislocations. For this reason, a design concept for a layered-structure of the nitride based semiconductor device is largely different from the other semiconductor devices. Normally, it is preferable that the active layer of the semiconductor laser diode is uniform in compositional profile and in energy band gap profile. If the active layer of the semiconductor laser diode is not uniform in compositional profile and in energy band gap profile, then the photo-luminescent efficiency is dropped, resulting in an undesired multiple wavelength laser emission. In the nitride semiconductor laser diode, however, the active layer has many defects. Carriers are likely to be captured by the defects and non-luminescence recombination is likely to appear at the defects. In order to avoid this problem, it is effective to form in-plane fluctuations of the potential of the active layer, so that the carriers are localized in the potential valleys provided by the potential fluctuations. If the carriers are localized in the potential valleys, then it unlikely appears that the carriers are captured by the defects and non-luminescence recombination appears at the defects. Differently from the other semiconductor devices, a non-uniform compositional profile of the active layer is preferable for the nitride semiconductor laser diode.
In general, the active layer of the gallium nitride based semiconductor device is made of InAlGaN which is hard to be grown in amorphous state, wherein a phase separation between InN and GaN or AlN is likely to appear. For this reason, the InAlGaN active layer is non-uniform in indium composition. This phase separation is naturally formed, and the in-plane potential fluctuation is formed in the active layer, thereby suppressing the non-luminescent recombination of carriers, resulting in an improvement in the photo-luminescent efficiency and in the reduction of the threshold voltage.
The above described technical matters are disclosed in Japanese laid-open patent publication No. 10-12969. This Japanese laid-open patent publication also describes as follows. InGaN is hard to be grown in amorphous state and a high tendency of phase separation of InN and GaN is shown. The in-plane non-uniformity in the indium composition of the quantum well layer causes that the quantum well layer is non-uniform in band gap profile, wherein a high indium composition region has a low potential energy and a low indium composition region has a high potential energy. Electrons and holes are localized in the high indium composition region having a low potential energy, whereby localized excitons are formed. The localized excitons drops the threshold value of the laser diode and increases the output.
The following similar technical matters are also disclosed in Applied Physics Letters, vol. 71, p. 2346, 1997. The INGaN is hard to be grown in amorphous state due to phase separation. The InN composition is fluctuated in the InGaN quantum well layer. Quantum disks or quantum dots restrict motion of excitons, whereby non-luminescent recombination is suppressed. A large fluctuation in the indium composition is effective to suppress the non-luminescent recombination and improve the luminescent efficiency.
The following similar technical matters are also disclosed in Applied Physics Letters, vol. 70, p. 983, 1997. The indium compositional fluctuation of the InGaN quantum well structure is observed on the basis of a cross sectioned transmission electron microscope photograph. Localization of exceptions suppresses the non-luminescent path, whereby a high quantum efficiency of the InGaN based laser diode can be obtained.
In case of the semiconductor laser diode having the InGaN quantum well layer, the phase separation of InN and GaN is likely to be caused in the InGaN layer. This phase separation causes the indium composition fluctuation which improve the luminescent efficiency, the threshold value and the laser output.
The indium compositional fluctuation in the active layer causes the fluctuation or non-uniformity in the energy band gap profile in the active layer, whereby a multiple wavelength laser emission is caused and a variation in photo-luminescent wavelength distribution due to injection current is caused.
In Japanese laid-open patent publication No. 11-340580, it is discussed that in order to avoid the above problem, it is effective to realize the uniformity in composition of the active layer, which is measured by a photo-luminescence peak wavelength distribution. The compositional uniformity is suppressed within xc2x10.03 to obtain a photo-luminescence peak wavelength distribution of not more than 150 meV, thereby suppressing the multiple wavelength laser emission.
In recent years, the requirement for improving the life-time of the nitride based semiconductor laser diode has been on the increase. If the nitride based semiconductor laser diode is applied to a light source for the next generation optical storage device such as digital video disk, then at least 5000 hours or longer life-time is necessary, wherein the life-time is measured by an APC examination at 70xc2x0 C. and 30 mW.
In Physica Status Solidi (a) vol. 176, p. 15, 1999, it is disclosed that reduction in dislocation density of substrate is effective for improving life-time of the laser diode. The laser diode uses a substrate with a reduced dislocation density and AlGaN/GaN modulation-doped cladding layer. If the APC examination is carried out at room temperature and 2 mW, then the life-time of not less than 10000 hours can be obtained. If, however, the APC examination is carried out at 60xc2x0 C. and 30 mW, then the obtained life-time is only 400 hours. This conventional laser diode does not satisfy the above requirement.
A recently developed method xe2x80x9cfacet-initiated epitaxial lateral growthxe2x80x9d is disclosed in Applied Sysics, vol. 68-7, 1999, pp. 774-779. This method is effective to obtain a GaN substrate with a largely reduced dislocation density. FIG. 1 is a cross sectional elevation view illustrative of a conventional gallium nitride based semiconductor laser diode over an n-GaN substrate with a low surface dislocation density which is prepared by the facet-initiated epitaxial lateral growth. An n-type cladding layer 102 is provided on a top surface of the n-GaN substrate 101, wherein the n-type cladding layer 102 comprises an Si-doped n-type Al0.1Ga0.9N layer having a silicon impurity concentration of 4xc3x971017 cmxe2x88x923 and a thickness of 1.2 micrometers. An n-type optical confinement layer 103 is provided on a top surface of the n-type cladding layer 102, wherein the n-type optical confinement layer 103 comprises an Si-doped n-type GaN layer having a silicon impurity concentration of 4xc3x971017 cmxe2x88x923 and a thickness of 0.1 micrometer. A multiple quantum well layer 104 is provided on a top surface of the n-type optical confinement layer 103, wherein the multiple quantum well layer 104 comprises two In0.2Ga0.8N well layers having a thickness of 4 nanometers and Si-doped In0.05Ga0.95N potential barrier layers having a silicon impurity concentration of 5xc3x971018 cmxe2x88x923 and a thickness of 6 micrometers. A cap layer 105 is provided on a top surface of the multiple quantum well layer 104, wherein the cap layer 105 comprises an Mg-doped p-type Al0.2Ga0.8N layer. A p-type optical confinement layer 106 is provided on a top surface of the cap layer 105, wherein the p-type optical confinement layer 106 comprises an Mg-doped p-type GaN layer having a magnesium impurity concentration of 2xc3x971017 cmxe2x88x923 and a thickness oil 0.1 micrometer. A p-type cladding layer 107 is provided on a top surface of the p-type optical confinement layer 106, wherein the p-type cladding layer 107 comprises an Mg-doped p-type Al0.1Ga0.9N layer having a magnesium impurity concentration of 2xc3x971017 cmxe2x88x923 and a thickness of 0.5 micrometers. A p-type contact layer 108 is provided on a top surface of the p-type cladding layer 107, wherein the p-type contact layer 108 comprises an Mg-doped p-type GaN layer having a magnesium impurity concentration of 2xc3x971017 cmxe2x88x923 and a thickness of 0.1 micrometer. Those layers 102, 103, 104, 105, 106, 107, and 108 were formed by a low pressure metal organic vapor phase epitaxy method under a pressure of 200 hPa. A partial pressure of the ammonium gas for nitrogen source was maintained at 147 hPa. TMG was used for the Ga source material. TMA was used for the Al source material. TMI was used for the In source material. The growth temperature was maintained at 1050xc2x0 C. except when the InGaN multiple quantum well active layer 104 was grown. In the growth of the InGaN multiple quantum well active layer 104, the growth temperature was maintained at 780xc2x0 C. A dry etching process was then carried out to selectively etch the p-type cladding layer 107 and the p-type contact layer 108 thereby forming a mesa structure 109. A silicon dioxide film 110 was formed on the mesa structure 109 and the upper surfaces of the p-type contact layer 108. The silicon dioxide film 110 was selectively removed from the top surface of the mesa structure 109 by use of an exposure technique, whereby the top surface of the p-type contact layer 108 was shown and a ridged structure was formed. An n-type electrode 111 was formed on a bottom surface of the substrate 101, wherein the n-type electrode 111 comprises laminations of a titanium layer and an aluminum layer. A p-type electrode 112 was formed on a top surface of the p-type contact layer 108, wherein the p-type electrode 112 comprises laminations of a nickel layer and a gold layer. The above structure was then cleaved to form first and second facets. The first facet was then coated with a highly reflective coat of a reflectance factor of 95%, wherein the highly reflective coat comprises laminations of titanium dioxide film and silicon dioxide
The obtained threshold current density was 3.7 kA/cm2, and the threshold voltage was 4.7V. The APC examination to the laser diode was carried out at 70xc2x0 C. and 30 meV The averaged life-time was 200 hours.
The conventional nitride based semiconductor laser diodes do not satisfy the requirement for not less than 5000 hours, when the APC examination to the laser diode was carried out at 70xc2x0 C. and 30 meV
In the above circumstances, it had been required to develop a novel nitride based semiconductor device free from the above problem.
Accordingly, it is an object of the present invention to provide a novel nitride based semiconductor device free from the above problems.
It is a further object of the present invention to provide a novel nitride based semiconductor laser diode improved in life-time under high temperature and high output conditions.
It is a still further object of the present invention to provide a novel nitride based semiconductor laser diode with a high photo-luminescent efficiency.
It is yet a further object of the present invention to provide a novel method of forming a novel nitride based semiconductor device free from the above problems.
It is still more object of the present invention to provide a novel method of forming a novel nitride based semiconductor device improved in life-time under high temperature and high output conditions.
It is yet more object of the present invention to provide a novel method of forming a novel nitride based semiconductor device with a high photo-luminescent efficiency.
The first present invention provides a semiconductor device comprising: a base layer made of a gallium nitride-based material; a cladding layer extending over the base layer; and an active layer extending over the cladding layer, and the active layer including at least a photo-luminescent layer of InxAlyGa1xe2x88x92xxe2x88x92yN (0 less than x less than 1, 0xe2x89xa6yxe2x89xa60.2), wherein a standard deviation xcex94x of a microscopic fluctuation in an indium composition of the photo-luminescent layer is not more than 0.067.
The second present invention provides a semiconductor device comprising: a base layer made of a gallium nitride-based material; a cladding layer extending over the base layer; and an active layer extending over the cladding layer, and the active layer including at least a photo-luminescent layer of InxAlyGa1xe2x88x92xxe2x88x92yN (0 less than x less than 1, 0xe2x89xa6yxe2x89xa60.2), wherein a standard deviation "sgr" of a microscopic fluctuation in a band gap energy of the photo-luminescent layer is not more than 40 meV.
The third present invention provides a semiconductor device comprising a base layer made of a gallium nitride-based material; a cladding layer extending over the base layer; and an active layer extending over the cladding layer, and the active layer including at least a photo-luminescent layer of InxAlyGa1xe2x88x92xxe2x88x92yN 0 less than x less than 1, 0xe2x89xa6yxe2x89xa60.2), wherein a differential gain xe2x80x9cdg/dnxe2x80x9d of the active layer satisfies dg/dnxe2x89xa71.0xc3x9710xe2x88x9220 (m2).
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.